Plastic parts are commonly welded by pressing the lower face of a horn or head against an upper surface of the part, to transmit sonic energy that welds adjacent surfaces of the parts. The lower surface of the head and the upper surface of the part must contact one another along all areas of the part, which generally requires the head and part surfaces to be precisely coplanar. The head is moveable up and down on a welding machine over a base of the machine. Although the lower surface of the head is approximately parallel to the upper surface of the base, they may be out of alignment by more than 1.degree.. The head and part surfaces commonly must be coplanar within an angle on the order of magnitude of 1/10th degree.
The part-holding part of the fixture may be tilted slightly by a mechanism that includes a ball that supports the upper assembly of the fixture on the lower one. When the upper assembly is in the precise orientation required, several bolts that lie around the ball and that connect the upper and lower assemblies, are tightened. This is a time consuming procedure that often leads to inaccuracies. A fixture that enables the upper assembly of the fixture to be readily tilted to the proper orientation and then easily locked in place, would be desirable.